JC Virus, a member of the Polyomaviridiae family, is the well-established etiological agent of Progressive Multifocal Leukoencephalopathy (PML), a fatal demyelinating disease of the Central Nervous System (CNS), frequently seen in patients with underlying immunosuppressive conditions. After the AIDS pandemic, cases of PML, once considered a rare disease associated with leukemias and lymphomas, has dramatically ncreased and now, PML is considered and AIDS defining condition. After primary infection the virus remains in latent state most likely in the kidney, and under immunosuppression enters the brain and efficiently replicates in oligodendrocytes, the myelin producing cells of the CNS ad abortively infects astrocytes. The lytic destruction of oligodendrocytes and the activation of astrocytes in response to injury caused y JCV infection, result in the characteristic histopathological landmarks of PML, extensive areas of myelin loss, in which numerous bizarre astrocytes with atypical and pleomorphic nuclei, and enlarged oligodendrocytes harboring intra-nuclear eosinophilic inclusion bodies can be found. The natural response to dispose of damaged or infected cells is programmed cell death. The intrincate mechanisms that will decide between cell death and cell survival depend on the delicate balance between pro-apoptotic proteins and inhibitor of apoptosis. Our preliminary immunohistochemical data in brain samples from patients with PML revealed increased expression of Survivin, a member of the inhibitors of apoptosis family, in bizarre astrocytes and in the intra-nuclear inclusion bodies of JCV infected oligodendrocytes. This protein is abundantly expressed during development in embryonic proliferating tissues, but is absent in terminally differenciated cells. It is believed that the mechanism of apoptosis inhibition involves the inactivation of caspases, the programmed cell death executioner proteins. Our results also show that JCV infected primary oligodendroglial cell cultures show enhanced expression of Survivin by Western blot and immunocytochemistry. In addition cell cycle analysis of infected cells demonstrated a reduction in the number of apoptotic cells and siRNA inhibition of Survivin resulted in a dramatic increase in apoptosis. These results suggest that JCV infection stimulates the expression of Survivin, which prevents apoptosis, and stimulates cell survival in order to compete its infectious cycle. [unreadable] [unreadable] [unreadable]